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Chapter 11
Watch Timer
Preliminary User’s Manual U15839EE1V0UM00
11.4.2 Control of the watch timer
The watch timer operates with time intervals from 500 µs to 16.4 s.
The watch timer generates at its overflow the INTWT interrupt request at fixed time intervals.
With the WTM1 bit and the WTM0 bit the watch timer function can be started.
With the WTM1 bit the watch timer function can be stopped independently from the interval timer
function.
For synchronous watch and interval timer function operation:
The count operation of the watch timer is started when the WTM0 bit and the WTM1 bit of the watch
timer mode control register (WTM) are set to “1”.
Both prescalers are stopped and cleared if the WTM0 bit is set to “0“.
For independent start or stop of watch timer function operation:
This functionality is only available, when the 11-bit Prescaler is running, too.
The count operation of the watch timer is started when the WTM1 bit and the WTM0 bit of the watch
timer mode control register (WTM) are set to “1”.
The WTM0 bit has to be set to “1” either it was “1” before. In that case the frequency of the running
11-bit prescaler is not influenced.
The 5-bit watch timer function prescaler is stopped and cleared if the WTM1 bit is set to “0“.
Caution:
If the 5-bit watch timer function prescaler is clocked by f
W
/2
9
(WTM3 = 0).
This prescaler is started with the next edge of the f
W
/2
9
clock. Therefore if the 11-bit
prescaler was running before the 5-bit prescaler watch timer is started, the INTWT
interrupt is generated up to one f
W
/2
9
period later then the time that the WTM1 bit was
set to “1”.
For example: if f
W
= f
XXT
/32
(i.e. f
XXT
= 32000 Hz) this can be up to 1 ms.
This happens only at the first starting edge of the f
W
/2
9
clock.
Table 11-3:
Example for Interval Time of Watch Timer
Remarks: 1. f
X
= 4 MHz
2. WTSEL0 bit = 1(CKC register)
→
f
CKSEL2
= f
XXT
/32
3. f
W
: Watch timer clock frequency
4. interval times change accordingly if f
X
= 4 MHz
WTM3
WTM2
Interval Time
f
W
= f
CKSEL2
=
f
X
/4096
f
W
= f
CKSEL1
=
f
XT
(32 KHz)
WTSEL1 = 0
WTSEL1 = 1
1
1
2
4
× 1/f
W
16.4 ms
0.5 ms
1
0
2
5
× 1/f
W
32.8 ms
1.0 ms
0
1
2
13
× 1/f
W
8.4 s
256 ms
0
0
2
14
× 1/f
W
16.8 s
512 ms
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